Geographic and Magnetic Poles

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Geographic and Magnetic Poles

Earths geographic poles are fixed by the axis of Earths rotation. On maps, the north and south geographic poles are located at the congruence of lines of longitude. Earths geographic poles and magnetic poles are in different places. As are all points on Earth, the northern magnetic pole is south of the northern geographic pole (located on the polar ice cap) and is presently located near Bathurst Island in northern Canada (approximately 1,000 miles [1,600 km] from the geographic North Pole). The southern magnetic pole is displaced hundreds of miles away from the southern geographic pole on the Antarctic continent.

Although fixed by the axis of rotation, the geographic poles undergo slight wobblelike displacements in a circular pattern that shifts the poles approximately six meters per year. Located on shifting polar ice, the North Pole (geographic pole) is technically defined as that point 90° N latitude, 0° longitude (although, because all longitude lines converge at the poles, any value of longitude can be substituted to indicate the same geographic point. The South Pole (geographic pole) is technically defined as that point 90° S latitude, 0° longitude. Early explorers used sextants and took celestial readings to determine the geographic poles. Modern explorers reply on global positioning system (GPS) coordinates to accurately determine the location of the geographic poles.

Earths magnetic field shifts over time, eventually completely reversing its polarity. There is evidence from magnetic mineral orientations suggesting that, during the past 10-15 million years, reversals have occurred as frequently as every quarter million years. Although Earths magnetic field is subject to constant change (periods of strengthening and weakening) and the last magnetic reversal occurred approximately 750,000 years ago, geophysicists assert that the next reversal will not come within the next few thousand years. The present alignment means that at the northern magnetic pole, a dip compass (a compass with a vertical swinging needle) points straight down. At the southern magnetic pole, the dip compass needle would point straight up or pole.

The magnetic poles are not stationary. The north magnetic pole migrates about 6 mi (10 km) per year. As igneous rocks cool from a hot magma any magnetic minerals will align themselves with the magnetic polarity present at the time of cooling. These rocks preserve a history of magnetic reversals and, when found in equidistant banded patterns on either side of mid-ocean ridges, provide paleomagnetic evidence of plate tectonics.

Navigators using magnetic compass readings must make corrections both for the distance between the geographic poles and the magnetic poles, and for the shifting of the magnetic poles. Moreover, the magnetic poles may undergo displacements of 25-37 mi (40-60 km) from their average or predicted position due to magnetic storms or other disturbances of the ionosphere and/or Earths magnetic field. Angular corrections for the difference between the geographic poles and their corresponding magnetic pole are expressed as magnetic declination. The values for magnetic declination vary with the observers position.

See also Bowens reaction series; Cartography; Continental drift; Earths interior; Global Positioning System; Latitude and longitude; Magnetism.